Referring to U.S. Pat. No. 5,362,282, “master chain link” is disclosed, in that a reusable master chain link for a derailleur chain is provided, which includes first and second parallel pins extending between first and second parallel plates. The first pin includes first and second heads at opposite ends thereof and the second pin also includes first and second heads at opposite ends thereof. The first and second plates have respective first and second apertures formed therein. The first and second apertures have respective narrow circular portions narrower than the respective first heads of the first and second pins. The narrow circular portions further include respective countersinks formed therein for receiving and substantially concealing the respective first heads of the first and second pins and substantially concealing the first heads within the respective narrow circular portions. The first and second apertures further include respective wide circular portions wider than the respective first heads of the first and second pins. The respective wide circular portions are connected to the respective narrow circular portions by an elongated slot to permit movement of the respective first heads therebetween. The respective second heads of the first and second pins are integrally connected by laser welds to the respective second and first plates.
Above mentioned structure provides the countersinks for concealing the first heads of the first and second pins, it still has some drawbacks. First, the prior art doesn't overcome the problem of disengagement. Due to the lack of force pushing the plate outward the first head won't engage with the countersink. The first head will still move between the narrow circular portion and the wide circular portion. This will increase the possibility of disengagement. Second, the prior art has the problem of strength. The cutting-off area of the plate is increasing due to the elongated slot. The strength of the plate will decrease.